The present invention relates generally to a method for forming a semiconductor device, and more particularly, to a method for forming a bulb-shaped structure in a recess gate. The bulb-shaped recess is capable of preventing an etching process margin reduction that causes a spacer oxide film formed on sidewalls of a recess to thickly laminate over a lower part of the recess.
High integration trends for semiconductor devices have brought problems. These problems include a narrowed gate line width and a reduced channel length, which led to deterioration in electrical properties of the semiconductor devices. A recess gate technology has been introduced to resolve such disadvantages. The recess gate is obtained by etching a semiconductor substrate at an expected gate region to a designated thickness, thereby increasing a contact area between an active region and the recess gate to increase the channel length.
However, as the miniaturization of semiconductor devices continues, forming a recess in a very small semiconductor device has become a rather difficult task. To secure an effective area of a recess, the lower part of the recess therefore was formed in a globular shape. That is, a method for forming a bulb-shaped recess was developed to increase the effective area of a recess.
In order to form a bulb-shaped recess, first of all, a hard mask pattern exposing an expected gate region is formed over a semiconductor substrate. The semiconductor substrate is then etched using the hard mask pattern as an etching mask, so as to form a recess.
Next, a spacer dielectric layer is formed on an entire upper surface of the semiconductor substrate including the recess.
The spacer dielectric layer formed at a lower part of the recess is removed by an anisotropic etching process, so that the spacer dielectric layer may remain only on sidewalls of the recess.
With the hard mask pattern and the remaining spacer dielectric layer as a mask, the lower part of the semiconductor substrate is isotropically etched to form a bulb-shaped portion.
In this manner, a bulb-shaped recess with the bulb-shaped portion formed on the lower part of the recess is obtained. The channel length of a gate thus can be increased accordingly.
Then, the hard mask pattern and the spacer dielectric layer are removed, and a gate is formed over the bulb-shaped recess.
However, the problem with this method is that the spacer dielectric layer may not be evenly formed on the upper surface of the semiconductor substrate, but tends to be formed thickly on the lower part of the recess. Because of this, the spacer dielectric layer may remain in the lower part of the recess even though the spacer dielectric layer is subsequently anisotropically etched.
The residual spacer dielectric layer on the lower part of the recess may reduce the process margin for performing the isotropic etching process required for forming a subsequent bulb-shaped portion, and may disturb the formation of a normal bulb shape.